Obesity affects a growing number of the U.S. population and often is closely associated with insulin resistance, type 2 diabetes, cardiovascular disease, and dyslipidemia. Obesity itself is typically a heterogeneous condition, due to regional distribution of fat tissue. Central obesity generally refers to fat accumulation in omental or visceral cavity, whereas peripheral obesity generally refers the subcutaneous fat accumulation. Epidemiological studies have established that central obesity is associated with a higher degree of risk than peripheral obesity to the above-mentioned diseases, however, the underlying mechanism(s) are generally not well understood. Presumably, distinctive biological properties of omental fat, in addition to its unique anatomical location, contribute to the increased pathogenicity of central obesity. It has been discovered that an excess of cortisol can cause central obesity and that treatment of HIV patients with protease inhibitor can lead to accumulation of omental fat accumulation but depletion of subcutaneous fat. In vitro studies have also demonstrated that abdominal visceral fat pads can be relatively resistant to the anti-lipolytic effect of insulin and susceptible to the lipolytic effect of catecholamine. At a molecular level, omental fat has been shown to have increased gene expression or secretion of interleukin 6, plasminogen activator inhibitor (PAI-1), and angiotensinogen, compared to subcutaneous fat. These observations indicate the existence of biological difference between omental and subcutaneous fat depots.
Adipose, i.e., fat, tissue plays a critical role in the pathogenesis of obesity and its associated diseases, but the molecular mechanisms for these associations generally remain unclear. Adipose tissue is generally recognized as an important endocrine organ that communicates actively with the central nervous system and other peripheral tissues through the release of a variety of bioactive factors that regulate glucose and lipid homeostasis. These factors, collectively known as adipocytokines, include leptin, tumor-necrosis factor α (TNFα), plasminogen activator inhibitor-1 (PAI-1), adiponectin/ACRP30/adipoQ, and resistin. Adipocytokines have been demonstrated to play a key role in the pathogenesis of obesity and its associated diseases. Nevertheless, current knowledge of known genes generally cannot fully explain the pathophysiology of obesity, and effective treatment for these diseases is still lacking.
Adipose tissue also plays a central role in energy homeostasis. Its primary function is to store and mobilize energy in the form of triglycerides in response to caloric excess and deprivation, respectively. Adipose tissue can be divided into white adipose tissue (WAT) and brown adipose tissue (BAT). WAT and BAT are different in morphology and biological function; WAT is monocular and mainly functions in storing triglyceride, while BAT is multilocular, rich in mitochondria, and designed to burn energy. BAT is mainly present in hibernating animals, rodents and new-born humans, indicating evolutional adaptation of adipose tissue to environment. Because the significance of the BAT to adult physiology is relatively not clear, references herein to adipose tissue or fat cells generally refer to WAT. Central obesity generally refers to intra-abdominal fat accumulation in visceral or omental adipose tissues, although, strictly speaking, omental fat is a subset of visceral fat.
Obesity is due to the excess accumulation of triglyceride in intra-abdominal (omental) and subcutaneous adipose tissue. Studies of animal models have provided some mechanistic understandings of obesity, likely a communication disorder between the central nervous system and the peripheral tissues, particularly adipose tissue. In mice, certain single gene mutations, such as db/db (leptin receptor mutation) and Ay (the agouti protein is homologous to melanocyte stimulating hormone), can cause obesity, suggesting that a defect involving the central nervous system is the cause for the obesity in these animals. Leptin mutations also can cause obesity in mice (ob/ob), and in humans. However, obesity caused by single gene mutations in humans is rare, and obesity in the general population is thought to be of polygenic origin.
The association of type 2 diabetes with obesity has been observed for a long time. Evidence from epidemiological, clinical and experimental studies has demonstrated that obesity is generally the greatest risk factor for insulin resistance and type 2 diabetes and, moreover, visceral obesity is associated with a higher degree of risk than peripheral obesity. The mechanism for the close association is generally not well understood, but it is generally accepted that an excess of fat leads to increasing insulin resistance and/or impaired glucose disposal, which can predispose someone to type 2 diabetes. The pancreas, liver, muscle, fat tissue, and central nervous system are the principal organs involved in regulating glucose and fat metabolism and are likely to participate in the pathogenesis of obesity and type 2 diabetes. However, recent experimental studies indicate that fat tissue can play a relatively major role in the etiology of type 2 diabetes. For example, surgical excision of visceral fat tissue in the rat has been shown to increase insulin sensitivity suggesting that excess fat is a causative factor for type 2 diabetes. In addition, lipodystrophic patients and fat-depleted mice have developed hyperinsulinemia and type 2 diabetes, and surgical implantation of adipose tissue reverses the diabetic phenotype. Also, adipocyte size may be a determinant of body insulin sensitivity, as it has been proposed that small adipocytes confer insulin sensitivity while large ones result in insulin resistance. These and other studies strongly support the premise that adipose tissue can play a central role in the regulation of insulin sensitivity, and in the pathogenesis of type 2 diabetes.
As discussed above, fat cells generally play an active role in energy storage, fatty acid metabolism and glucose homeostasis. To perform this specialized function, the adipocyte expresses a special subset of genes to communicate with the central nervous system and peripheral tissues, and to respond to various neuronal, metabolic and hormonal signals. The adipocyte secretes a number of bioactive substances, collectively known as adipocytokines, such as, for example, leptin, TNFα, PAI-1, adiponectin, and resistin. These adipocytokines function as endocrine, paracrine, and autocrine factors, and have been implicated in obesity and its associated diseases. Some of these adipocytokines are discussed briefly below.
Leptin is a hormone secreted from fat tissue into the circulation that acts to reduce food intake and increase energy expenditure mainly through binding to leptin receptors in the hypothalamus. Leptin secretion is regulated by the energy supply; starvation decreases its expression and secretion, while overfeeding or increased adiposity induces leptin expression. Leptin is therefore a key molecule linking this adipose tissue to the central nervous system and regulating energy homeostasis.
Tumor necrosis factor-alpha (TNFα) is a cytokine produced not only by inflammatory cells but also by adipocytes. TNFα expression has been shown to be elevated in the fat tissue of obese animals and humans. TNFα appears to induce insulin resistance by interfering directly and/or indirectly with insulin signaling pathways in an autocrine or paracrine fashion. The absence of TNFα results in significantly improved insulin sensitivity in obese mice, the mice lacking TNFα receptors appears protected against diabetes to a certain degree, implying that there might be a yet uncharacterized pathway involved in TNFα-induced insulin resistance.
Plasminogen activator inhibitor-1 (PAI-1) is a key pathogenic factor for thrombotic vascular disease. Plasma PAI-1 levels are closely correlated with visceral fat, and gene expression is highly elevated in visceral fat during the development of obesity. TNFα has been shown to induce adipose PAI-1 expression, providing a possible explanation for the association of obesity with cardiovascular disease.
Adiponectin is a hormone secreted exclusively from adipose tissue and is also referred to as ACRP30, AdipoQ, apMI, or GBP28. Adiponectin has been demonstrated to have promising activities potentially for the treatment of obesity and diabetes. Its expression is reduced in the states of obesity and type 2 diabetes, and its replenishment improves insulin sensitivity and prevents diet-induced obesity in rodents, probably by increasing fat oxidation and decreasing triglyceride content in muscle and liver. This effect can result from increased expression of molecules involved in both fatty-acid combustion and energy dissipation in muscle. The mechanisms for these actions are generally not clear. Adiponectin consists of collagenous repeats and a globular domain homologous to complement C1q, and shares structural similarity to TNFα. Interestingly, PPARγ induces, whereas TNFα suppresses, the expression and secretion of adiponectin, suggesting that adiponectin may be a target molecule relaying insulin sensitivity.
Resistin is a hormone typically isolated from differentiated 3T3-L1 adipocytes by screening for genes regulated by the PPARγ agonist rosiglitazone. During adipocyte differentiation, resistin is increasingly expressed but is suppressed by treatment with rosiglitazone. Moreover, ob/ob mice secrete increased amounts of resistin, and recombinant resistin induces insulin resistance. Resistin has therefore been proposed to be a link between obesity and insulin resistance. However, conflicting results have been reported, in which resistin expression was reduced in several obese animal models and was induced by PPARγ agonists. In addition, unlike the high expression of mouse resistin in adipose tissue, the expression of the human counterpart is very low.
There are additional adipocyte-specific/abundant genes, such as is adipsin and angiotensin, acylation stimulating protein, PGAR, and interleukin-6, whose functions in obesity and type 2 diabetes are generally less understood. Nevertheless, the discovery of a myriad of adipose secreted factors has generally established adipose tissue as an endocrine organ. The dysregulation of adipose tissues autocrine, paracrine and endocrine function is likely to disturb energy homeostasis and lead to obesity, type 2 diabetes, dyslipidemia and hypertension.
Abdominal fat is generally more pathogenic than subcutaneous fat. An obvious explanation for this may simply relate to its anatomical location. Visceral adipose tissue drains via the portal venous system, such that liver is fully exposed to and functionally affected by bioactive substances released from this depot. In addition, differences in physiology, biochemistry and gene expression have been observed between omental and subcutaneous fat tissues. Abdominal obesity is predominant in males whereas subcutaneous fat mass is mostly involved in female obesity, indicating that sex hormones may play a role in these differences. Moreover, an excess of cortisol is known to cause central obesity. Finally, a selective increase in visceral fat is a common feature of aging. It has been suggested that these two adipose tissue depots differ in important ways. Omental adipose fat is more metabolically active with respect to lipolysis and lipogenesis. Compared to subcutaneous fat, abdominal fat pads have greater secretion of interleukin 6, plasminogen activator inhibitor (PAI-1), angiotensinogen, and the rate of apoptosis is greater. In contrast, leptin expression is higher in subcutaneous fat tissue than omental fat tissue. Yet, whether these changes discussed above can explain features of insulin resistance syndrome generally remains unclear. Because the pathophysiological basis of this syndrome is likely to be complex, several genes/gene products and pathways may participate in the disease process.
Insulin signaling is a complex and coordinated process involving protein modification, translocation, and compartmentalization. Insulin action is initiated through binding of insulin to the α subunit of insulin receptor (IR), which activates the beta subunit intrinsic receptor tyrosine kinase, resulting in autophosphorylation of insulin receptor β subunit and tyrosine phosphorylation of intracellular target proteins such as IR substrates (IRS-1-4) and Shc, Cb1, Gab-1. Three major signaling pathways are initiated by these intracellular targets: 1) IRS/PI 3-kinase/Akt; 2) CAP/Cb1; and 3) Shc(or Gab)/Ras/MAP kinase.
In the first major pathway, tyrosine-phosphorylated IRS-1 or IRS-2 binds to src-homology 2 domains of intracellular proteins, including p85, a regulatory subunit of phosphatidylinositol 3-kinase (PI 3-kinase). The interaction of IRS and p85 subunits results in the activation of the p110 catalytic subunit of PI 3-kinase, which raises phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate (PIP3) levels. These second messengers activate phosphoinositide-dependent kinase-1 (PDK-1) to phosphorylate and hence activate Akt (also called protein kinase B) and atypical PKC isozymes.
In the second major pathway, c-Cbl-associated protein (CAP) recruits c-Cbl to the insulin receptor where it is phosphorylated. This protein complex subsequently localizes to lipid raft domains of the plasma membrane called caveola. The SH2-containing adapter protein CRKII and C3G, a guanine nucleotide exchange factor, are then targeted to phosphorylated c-Cbl at the lipid raft. C3G may activate TC 10, a G-protein of the rho family, which is expressed in adipose and muscle tissue. The IRS/PI 3-kinase/Akt and CAP/Cbl pathways are generally believed to function in concert to upregulate glucose transport in response to insulin.
The third major pathway involves the activation of the p42/44 MAP kinase (mitogen activated protein kinase) cascade. Insulin receptor phosphorylation of both Shc and Gab-1 adaptor proteins leads to Ras activation of multiple kinases resulting in activation of MAP kinase (Erk1 and 2). This pathway is more involved in the mitogenic function of insulin.
Many other factors interact with and modify the efficiency of insulin signaling in a positive or negative manner, which include protein kinases, e.g., AMP-activated kinase, protein kinase C, and IKKβ, phosphatases, e.g., PTP1B, SHIP2, PTEN, and modulators of IR activity, e.g., PC-1.
There is a need for methods of detecting and treating diseases of or relating to adipose tissue and glucose metabolism, such as obesity and type 2 diabetes.